1. Field of the invention
Oxide-dispersion-strengthened superalloys based on nickel which, owing to their outstanding mechanical properties at high temperatures, are used in the construction of heat engines. Preferred use as blade material for gas turbines.
The invention relates to the improvement of the mechanical properties of oxide-dispersion-strengthened nickel-base superalloys with altogether optimum properties in relation to high-temperature strength, long-term stability and ductility. In this connection, fatigue strength and good thermal shock behavior in the medium and high temperature range of the material are to the fore.
In the narrow sense, the invention is concerned with a process for producing coarse, longitudinally oriented column crystals with improved temperature change resistance and increased ductility in the transverse direction in a workpiece of any cross-sectional size and cross-sectional shape from an oxide-dispersion-strengthened nickel-base superalloy, which exists in the initial condition in fine-grained hot-worked form, by a coarse-grain annealing which initiates the secondary recrystallization.
2. Discussion of background
High-temperature blade materials for gas turbines such as oxide-dispersion-strengthened nickel-base superalloys are used in the state involving coarse, longitudinally directed column crystals. If the longitudinal axis of these directionally arranged crystallites coincides with the longitudinal axis of the workpiece and if the latter is at the same time the main stressing direction, optimum results in relation to creep strength and fatigue strength at high temperatures are achieved in this direction. The microstructural condition necessary for this is achieved by using a zone annealing process for the heat treatment which governs the secondary recrystallization with preferred direction. As a rule, the zone annealing is carried out in a conventional manner with comparatively limited cross-sectional dimensioning (a few cm.sup.2). If large cross-sectional dimensions (10 cm.sup.2 and over) are required, difficulties arise. Either the zone annealing cannot be carried out at all, the core zone failing to undergo coarse-grain recrystallization in the desired manner, or elaborate and complicated processes and apparatuses are necessary in order to reach the desired objective. In addition, the ductility in the transverse direction of the column crystals and the temperature change resistance leaves something to be desired.
The following literature is cited in relation to the prior art:
G. H. Gessinger, Powder Metallury of Superalloys, Butterworths, London, 1984 PA0 R. F. Singer and E. Arzt, "High Temperature Materials for Gas Turbines", Conf. Proc., Liege, Belgium, October 1986 PA0 J. S. Benjamin, Metall. Trans. 1970, 2943-2951 PA0 M. Y. Nazmy and R. F. Singer, Effect of inclusions on tensile ductility of a nickel-base oxide dispersion strengthened superalloy, Scripta Metallurgica, Vol. 19, pp. 829-832, 1985, Pergamon Press Ltd. PA0 T. K. Glasgow, "Longitudinal Shear Behaviour of Several Oxide Dispersion Strengthened Alloys", NASA TM-78973 (1978) PA0 R. L. Cairns, L. R. Curwick and J. S. Benjamin, Grain Growth in Dispersion Strengthened Superalloys by Moving Zone Heat Treatments, Metallurgical Transactions A, vol. 6 A, January 1975, pp. 179-188.
The known processes for producing longitudinally oriented column crystals in oxide-dispersion-strengthened nickel-base superalloys no longer meet the present requirements. The results achieved by these processes are no longer adequate for an optimum use of these materials. There is therefore a strong requirement for further development and improvement of the production processes.